Fix bug in the MapQTL plugin

The MapQTL plugin was ignoring the first marker in a linkage group, so if 
that marker had a QTL, it was not found.

Adjust unit-test accordingly
Fix bug in the MapChart output

The format of the data matrix has changed and MapChart was still relying
on the first format, ie: ignoring the first trait which was thus missing
from the generated output file

Adjust unit-test accordingly